Abstract
A leading theory for the biological effects of low-intensity magnetic fields is the spin-chemical Radical Pair Mechanism. This mechanism is best described by the master equation for the density matrix of an open quantum system, which includes terms for Hamiltonian evolution, thermal spin relaxation, and chemical kinetics. In this study, we have found a solution to this equation for a simplified radical pair model and shown that a significant magnetic effect occurs when the following condition is met: γ H τ 1 + κ τ, where γ is the gyromagnetic ratio of the electron, H is the magnetic field strength, τ is the relaxation time, and κ is the rate of chemical kinetics.